Saturday, June 01, 2013

A Little Nostalgia

In the decade that spanned the 1960s, the cost of computers fell by about two orders of magnitude, and with volume manufacture possible, they found their way into general scientific engineering and industrial use.

One area of science that particularly benefited from more widely available computing power, was that of biomedical research.  Two computer designs, in particular, spurred on this advancement,  the LINC of 1962, and the PDP-8 of 1965.

About 50 LINC machines were built, and a few survive today. This video shows a restored LINC in use.

These were the first "benchtop" designs, and set the trend in design for the machines that were to be known as minicomputers.  In 1965 the PDP-8 cost $18,000, still a huge amount of money, but cheap enough to be built in volume and satisfy the needs of university laboratories.

Both the LINC and the PDP-8, owed their design to the general purpose digital modules, that were manufactured by DEC from the late 1950s onwards. Each machine was a pared down 12 bit design, cost engineered for efficiency at every level, offering a machine capable of real work for a modest price.

Over the next 20 years, the PDP-8 architecture was to be revisited many times, as advancements in   logic ICs and memory allowed significant cost savings to be made.  The original PDP-8 was the size of a small fridge, within 10 years it was housed in a single 19" rack.  The very last PDP-8 machines used a microprocessor (Intersil/Harris 6120) to implement the CPU.

By the mid 1970s, microprocessors were starting to appear, which led to the era of 8-bit microcomputers.  Some of these, such as the MITS Altair 8800 paid homage to the style of the PDP-8 with a front panel of many LEDs and programming switches.  The simple LED indicators allowed the user to see precisely what the registers were holding, and the switches allowed simple programs to be "toggled" directly into memory.

These machines are now approaching 50 year old , yet there are many enthusiasts who started their scientific or engineering careers programming such computers.  The National Museum of Computing at Bletchley Park owns an early PDP-8, which they have restored to working order.  They have made a series of short videos depicting its operation.

Here's how a short program is loaded into the machine from the front panel switches.

Notice how much noise was present in a typical computer room.  Fans, printers and teletypes produces a near constant din.

The machines were relatively slow and simple by today's comparison,  capable of some 300,000 instructions per second. Nevertheless, they were capable of real work, despite their limitations in speed and memory.

In the mid 1960s, the standard user interface was a teletype printer with paper tape punch and reader. Programs would be loaded into the memory from paper tape, but often only after a simple tape loader had been manually toggled into the machine from the front panel switches.

The second of the NMOC videos shows how program on the the paper tape was loaded into memory.

Application programs had to fit within the 4K memory limitation.  This video shows a Chess program, and the user interaction using the ASR33 Teletype, that was supplied with every PDP-8.

Contemporary to the LINC and the PDP-8 was a low cost magnetic tape drive, commercialised by DEC as the DECtape.  The last of the NMOC videos, shows the DECtape in use

No comments: